The present invention relates to a display apparatus including a light-emitting device driven by current such as an organic electro luminescence (EL) device, to a source driver used in the display apparatus, and to a display panel.
In an active matrix type image display apparatus, a large number of pixels are arranged in a matrix pattern and the intensity of light is controlled for every pixel depending on provided luminance information, thereby displaying an image. For this purpose, a rectangular display panel, for example, includes thin-film-transistors (TFTs) arranged in a matrix pattern and controlling the state of liquid crystal or an optical material, source drivers provided along the upper and lower sides of the panel and gate drivers provided at the lateral sides of the panel.
Most of the conventional image display apparatuses such as display panels have used liquid crystal as an optical material. In each of these image display apparatuses, a liquid crystal driver as a source driver supplies display information in the form of voltages to respective pixels so that the transmissivities of pixels change depending on the display information.
On the other hand, image display apparatuses using organic EL devices as light-emitting devices have been intensively developed in recent years. Unlike liquid crystal, the organic EL devices emit light by themselves, so that display panels using the organic EL devices have the advantages of high visibility and the unnecessity of back lighting. The organic EL devices used for the display panels function as diodes and emit light upon the application of current.
FIG. 23 is a circuit block diagram schematically showing a configuration of a conventional organic EL display apparatus.
As shown in FIG. 23, the conventional organic EL display apparatus includes: a display panel; a pixel section 1005 provided on the display panel; a transmission path 1003 connected to the pixel section 1005; and a pixel driver 1001 included in a source driver and used for supplying a drive current to the pixel section 1005 via the transmission path 1003. The transmission path 1003 includes a line for connecting the source driver and the display panel to each other and a signal line provided on the display panel. The transmission path 1003 shown in FIG. 23 includes resistances and capacitances, which respectively indicate wiring resistances and stray capacitances.
The pixel driver 1001 includes a plurality of current sources. The sum of the currents flowing from current sources which are in a conductive state is supplied as an output current to the pixel section 1005 connected to the associated signal line.
The pixel section 1005 includes: a current generator 1011 having a pixel input capacitance 1007 and a current source 1008; and an organic EL device 1009 connected to the current source 1008. The “pixel section” shown in FIG. 23 is composed of three sub-pixel sections for displaying R (red), G (green) and B (blue), respectively, in reality.
Now, the configurations of the pixel driver and the pixel section and a black and white display of the organic EL display apparatus will be described.
FIG. 24A is an enlarged view showing a display panel in the case of a black and white display in the conventional organic EL display apparatus. FIG. 24B is a circuit diagram showing pixel sections arranged on the XXVb-XXVb line on the display panel shown in FIG. 24A and pixel drivers connected to the respective pixel sections. FIG. 24C is a graph showing an operating point of a TFT in a black display mode. FIG. 24D is a graph showing an operating point of the TFT in a white display mode.
As shown in FIG. 24B, a plurality of pixel drivers shown in FIG. 23 are arranged in a source driver. Specifically, the conventional source driver includes: a first pixel driver 1001a1; a second pixel driver 1001a2; . . . ; an n-th pixel driver 1001an; and a reference current generator 1101 for generating a current to be supplied to the respective pixel drivers 1001.
The reference current generator 1101 includes: a first pMOSFET 1108 whose source receives a power-supply voltage; a resistance 1107 connected to the first MOSFET 1108 at one terminal and grounded at the other terminal; a second pMOSFET 1109 forming a current mirror together with the first MOSFET 1108; and a third nMOSFET 1110 whose drain is connected to the drain of the second MOSFET 1109 and whose source is grounded.
Each of the pixel drivers 1001 is composed of a plurality of current sources forming current mirrors together with the third MOSFET 1110 and switches connected to the respective current sources. For example, in a display apparatus producing a display of 64 levels of gray scale, the first pixel driver 1001a1 includes: a first current source 1112 for outputting a current I; a second current source 1113 for outputting a current 2I; third, fourth and fifth current sources (not shown) for outputting currents 4I, 8I and 16I, respectively; a sixth current source 1114 for outputting a current 32I; and switches 1115, 1116 and 1117 connected to the respective current sources. The current sources are composed of nMOSFETs forming current mirrors together with the third MOSFET 1110.
Each of the sub-pixel sections of the pixel section 1005 shown in a simplified manner includes: an organic EL device 1009; a first TFT connected to the pixel driver 1001; and a second TFT forming a current mirror together with the first TFT and used for supplying a current input to the first TFT to the organic EL device 1009. In this example, the TFTs on the panel are pMOSFETs, so that a current is drawn from a pixel into a pixel driver in actual operation.
In a case of a black and white display as shown in FIG. 24A, all the switches in the pixel driver 1001a1 are OFF and a pixel section 1005a1 producing a black display is charged by the power supply voltage. In this case, as shown in FIG. 24C, even when the voltage at the output terminal of the source driver is high, a current flowing is very small. The point of intersection of the IV (current·voltage) curve of the TFT and the I-V characteristic curve of output of the source driver is the operating point of the TFT.
On the other hand, with respect to a pixel section 1005an producing a white display, all the switches in the pixel driver 1001a1 are ON, so that charge is drawn from the pixel section 1005an into the pixel driver 1001an. In this case, as shown in FIG. 24D, the operating point of the TFT shifts to lower potentials than in the case of the black display. The “black display” may be also referred to as “a low-luminance display” and the “white display” may be also referred to as “a high-luminance display”.
Now, specific examples of a configuration of the current generator 1011 shown in FIG. 23 will be described.
FIGS. 25A and 25B are circuit diagrams respectively showing examples of a configuration of a current generator in a general organic EL pixel section.
A current generator 1011 shown in FIG. 25A includes: a first switching transistor M4 connected to a pixel driver at one terminal; a second switching transistor M3 connected to the first switching transistor M4 in series; a capacitance C1 connected to the first and second switching transistors M4 and M3 in series and receiving a power-supply voltage at one terminal; a first p-channel TFT M2 whose drain is connected to a line connecting the first and second switching transistors M4 and M3 to each other and whose source receives a power-supply voltage; and a second TFT M1 forming a current mirror together with the first TFT M2 and having its drain connected to the organic EL device 1009. The line connecting the capacitance C1 and the second switching transistor M3 to each other is connected to the line connecting the gate electrodes of the first and second TFTs M2 and M1 to each other. Both the first and second switching transistors M4 and M3 are pMOSFETs in this example and have their operation controlled with control signals K1.
In the current generator 1011 shown in FIG. 25A, in a current setting mode, both the first and second switching transistors M4 and M3 are ON with the control signals K1 so that a current flows into the pixel driver 1001 and the capacitance C1 is charged by the gate voltage Vc1. When the capacitance C1 is charged, a constant current flows through each of the first and second TFTs M2 and M1. The “current setting mode” herein refers to a period from when a horizontal scanning period starts to when the current flowing in the pixel section 1005 reaches a target value.
In a display mode, both the first and second switching transistors M4 and M3 are OFF with the control signals K1. In this period, the gate voltage Vc1 is held by the capacitance C1, so that a current continuously flows from the second TFT M1 to the organic EL device 1009 in the same amount as that in the current setting mode.
A current generator 1011 shown in FIG. 25B includes: a first switching transistor M4 connected to a pixel driver 1101 at one terminal; a capacitance C1 receiving a power-supply voltage at one terminal and connected to the first switching transistor M4 at the other terminal; a second switching transistor M3 interposed between the first switching transistor M4 and the capacitance C1; a TFT M1 whose gate electrode is connected to the capacitance C1 and the second switching transistors M3, whose source receives a power-supply voltage and whose drain is connected to the organic EL device 1009; and a third switching transistor M5 interposed between the TFT M1 and the organic EL device 1009. The drain of the TFT M1 is also connected to the first and second switching transistors M4 and M3. The first and second switching transistors M4 and M3 are controlled with first control signals K1. The third switching transistor M5 is controlled with a second control signal K2, which is a signal of opposite phase to that of the first control signals K1.
In this current generator 1011 shown in FIG. 25B, in the current setting mode, both the first and second switching transistors M4 and M3 are ON with the first control signals K1 and the third switching transistor M5 is OFF with the second control signal K2. In this period, a current flows from the current generator 1011 to the pixel driver and the capacitance C1 is charged by the gate voltage Vc1. When the capacitance C1 is charged, a constant current flows into the TFT M1.
In the display mode, both the first and second switching transistors M4 and M3 are OFF and the third switching transistor M5 is ON. In this period, the gate voltage Vc1 is held by the capacitance C1, so that a current continuously flows from the TFT M1 to the organic EL device 1009 in the same amount as that in the current setting mode.